The protection of weatherable substrates has long been a desired goal. The ideal weatherable coating compositon will provide desirable protection against the elements by sufficiently filling holes and bridging gaps in irregularly surfaced substrates. Moreover, such a sealant composition must be readily applied and cured. Frequently, it is necessary that such a coating composition exhibit a good deal of flexibility, e.g., when applied to flexible substrates such as fibers, fabrics and the like. In the past, one of the preferred methods of achieving such protection has involved the formation of protective polymeric membranes upon the substrate to be protected
Among the polymeric materials which have been employed in the past for the protection of weatherable substrates are certain moisture-curable high molecular weight silane-grafted rubbery polymers. Thus, for example, U.S. Pat. No. 3,503,943 to Kresge et al discloses moisture curable polymers made by grafting halosilanes, particularly allyl trichlorosilane and vinyl trichlorosilane, onto ethylene/propylene/nonconjugated diene ("EPDM") terpolymer. However, the halosilane modified polymers made by Kresge et al possess such a high halosilane functionality per polymer chain (ranging from about 83 to 336) and per 1000 molecular weight of polymer (ranging from about 0.65-2.5 per 1000 Mv) that, when cured, the resulting polymeric membrane is highly crosslinked. Consequently, the compositions of this patent are not suitable for protecting highly flexible weatherable substrates. Moreover, it is noteworthy that all the graft copolymers actually made by Kresge et al have an inherent viscosity of more than 2.0 dl/g in tetralin and thus possess molecular weights of more than about 90,000.
Somewhat similarly, U.S. Pat. No 3,644,315 to Gardner et al discloses moisture-curable halosilane grafted conjugated diolefin polymers having a number average molecular weight of between about 3,000 and about 1,500,000. However, because such polymers possess unsaturation in their backbones, they are somewhat prone to degradation. Moreover, the graft polymers actually disclosed in this patent contain about 7.7-69 grafted silane groups per chain, and thus cure to form highly crosslinked, relatively inflexible materials. It is to be noted that the lowest molecular weight of any backbone polymer shown in the examples of Gardner et al has a molecular weight of more than 40,000.
Although Kresge et al and Gardner et al, discussed above, do state that lower molecular weight backbone polymers may be employed, these patents do not exemplify this statement. In this regard, the conclusions of the survey article presented by G. Wouters and F. Woods entitled "Moisture-Curable Silane Grafted Ethylene Propylene Elastomers" and presented at the International Rubber Conference 1981, Harrogate, U.K.--i.e., almost 10 years after the issuance of the Kresge et al and Gardner et al patents--is extremely noteworthy. Specifically, in Table 5 of their presentation, Wouters et al conclude that "EPM's or EPDM's with low molecular weight" are "structures with low moisture-curing potential."
Baldwin et al (in U.S. Pat. No. 3,366,612) discloses a method of rendering halogenated polymers (including low molecular weight liquid halogenated polymers) moisture-curable, which method comprises reacting such polymers with an aminosilane compound. No discussion of the cross-linking of non-halogenated polymers is present.
Consequently, it is totally unexpected that highly weatherable and, in many cases, flexible protective coatings could be formed from low molecular weight polymers having non-halogenated saturated carbon backbones.